Doped-ring amplifying optical fiber, and an amplifier containing such a fiber

ABSTRACT

The amplifying optical fiber comprises a single-mode core and a multimode core surrounding the single-mode core, the multimode core containing a doped layer referred to as a “doped ring” and having a certain concentration of active rare earth ions to perform amplification by active rare earth ions on at least one optical signal for injection into the amplifying fiber. The fiber is dimensioned so that the product of its length multiplied by its Raman efficiency is greater than or equal to 0.5 W −1 . In addition, the fiber presents absorption defined by an absorption coefficient expressed in dB/m, which absorption presents, at a certain wavelength, a maximum value referred to as the “absorption maximum”, the fiber presents accumulated absorption, corresponding to the product of its length multiplied by the absorption maximum, that is greater than or equal to 100 dB. The invention also provides an amplifier including such a fiber, a single-mode pump, and a multimode pump.

CROSS REFERENCE OF RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/791,382 filed Mar. 3, 2004, now U.S. Pat. No. 7,308,178, whichclaims benefit of French Application No. 03 02 602 filed Mar. 4, 2003,the entire disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to the field of optical fibertelecommunications. More precisely, the invention relates to adoped-ring amplifying optical fiber and to an amplifier containing sucha fiber.

In known manner, it is desired to increase the performance oferbium-doped fiber amplifiers (EDFAs), which amplifiers are generallyused on long-distance optical links in order to amplify wavelengthdivision multiplexed (WDM) signals.

The fibers are designed to obtain the most efficient amplificationpossible over a wavelength band that is as broad as possible or over aplurality of bands such as band C (1530 nanometers (nm) to 1565 nm) orband L (1565 nm to 1625 nm).

The document entitled “30% power conversion efficiency from a ring-dopedall-silica octagonal Yb-free double-clad fiber for WDM applications inC-band” by P. Bousselet et al., in Optical Amplifiers and TheirApplications Conference, PD1, 2001 Technical Digest, Optical Society ofAmerican, pp. 2-4, discloses an EDFA amplifier incorporating asilica-based amplifying optical fiber. More precisely, the optical fiberpresents the following structure:

-   -   a single-mode core, having a central region with a diameter        equal to 7 micrometers (μm);    -   a multimode core, being an intermediate region surrounding the        central region and having an outer diameter equal to 44 μm, the        multimode core containing a layer doped in active erbium ions,        referred to as a “doped ring”; and    -   cladding, an outer region surrounding the intermediate region        and having an outer diameter equal to about 150 μm.

The EDFA also includes a multimode pump of power equal to 2.3 watts (W),delivering a pump wave at a wavelength equal to 980 nm, and coupled tothe amplifying fiber by a single-mode fiber and a wavelengthmultiplexer. In order to be amplified, multiplexed optical signals ofwavelengths in band C are injected into the amplifying fiber.

The EDFA disclosed possesses gain in band C that is greater than that ofconventional EDFAs because of better conversion efficiency in saiderbium-doped ring amplifying fiber.

The object of the present invention is to devise a compact andintegrated fiber amplifier of low cost and having performance that isfurther improved in terms of efficiency, amplification level, number ofchannels (enlarging band C and/or L or new gain ranges . . . ), andamplified signal quality (low noise level, low dependency on variationsin pumping power, . . . ).

To do this, the invention seeks to make it possible to make a hybridamplifier making effective use both of the Raman effect and of theamplifying properties of active rare earth ions such as erbium ions. Theamplifier then benefits from the combined advantages of bothamplification techniques.

A few definitions of parameters involved in the invention are initiallyrecalled.

To a first approximation, the Raman efficiency Cr of a fiber is definedby the equation:Log(Pon/Poff)=Cr.Pp.L

in which Log is the Napierian logarithm, L is the length of the fiber,Pp is the power of a Raman pump wave injected into the fiber, Pon is theoptical power at the outlet of the fiber of an optical signal that hastraveled along the fiber in the presence of the pump wave, and Poff isthe optical power at the outlet of the fiber of the same optical signalthat has traveled along the fiber in the absence of the pump wave.

Another way of expressing the above relationship involves the Raman“on/off” gain, Gonoff, which, expressed in decibels (dB), can beestimated by the formula given below, which applies to an amplifier ofshort length and providing the attenuation of the Raman pump is weak(less than 2 decibels per kilometer (dB/km):Gonoff=10 log(Pon/Poff)=4.34 Cr.Pp.L

in which log is the base 10 logarithm.

Raman efficiency Cr is expressed per watt and per kilometer (W⁻¹km⁻¹)and can lie in the range 0.5 W⁻¹km⁻¹ to 5 W⁻¹km⁻¹, for example.

In addition, the presence of active rare earth ions in a fiber impliesthat the fiber absorbs an injected optical signal traveling along it. Ifan appropriate pump wave is injected, the fiber is also the seat ofstimulated emission, and the difference between emission and absorptionconstitutes the gain of the doped fiber. This absorption is defined byan absorption coefficient expressed in decibels per meter (dB/m) whichpresents a maximum value as a function of signal wavelength known as theabsorption maximum. This absorption maximum is also expressed in dB/mand is defined as the absorption peak. The absorption peak is obtainedfor a wavelength that also corresponds to the emission maximum and liesaround 1530 nm for erbium, for example.

Thus, the present invention provides a doped ring amplifying opticalfiber comprising:

-   -   a single-mode core of given diameter; and    -   a multimode core surrounding the single-mode core and containing        a doped layer referred to as a “doped ring”, having a certain        concentration of active rare earth ions, the fiber being        suitable, because of the active rare earth ions, for amplifying        an optical signal for injection into the amplifying fiber;

the fiber being characterized in that it is of a length and has Ramanefficiency such that the product of said length multiplied by said Ramanefficiency is greater than or equal to 0.5 W⁻¹, and in that, for saidfiber presenting absorption for an injected optical signal due to thepresence of active rare earth ions, said absorption being defined by anabsorption coefficient expressed in dB/m and presenting a maximum valueas a function of the wavelength of said signal, which value is referredto as the absorption maximum, said fiber presents accumulatedabsorption, corresponding to the product of said length multiplied bysaid absorption maximum, which is greater than or equal to 100 dB.

For a given Raman efficiency, the invention amounts to selecting awavelength that is long enough to obtain appreciable Raman gain, atlimited Raman pump power, and accumulated absorption adapted to thedesired level of rare earth ion amplification gain. The accumulatedabsorption condition also implicitly gives a limiting value on thelength of fiber that allows rare earth ion gain to exist, it beingunderstood that such gain diminishes and then disappears if the lengthof the fiber is increased beyond a certain limit.

Thus, the absorption maximum, the length of the fiber, and the Ramanefficiency are parameters that are interrelated, and they arejudiciously selected to make both types of amplification effective.

It is appropriate at least for the length of fiber to be selected to beshort enough to ensure that amplification by active rare earth ionsprovides gain of not less than 1 dB. Naturally, it is preferable toselect a length that provides gain that is much greater, it beingunderstood, for example, that erbium ion amplification makes it possibleunder such conditions to achieve gain of about 60 dB, by providingsufficient erbium pump power.

The two types of gain are cumulative in the sense that they can add whenthey act on a common range of wavelengths or they can enable opticalsignals to be amplified in two distinct ranges that do not overlap.

For example, the Raman gain may correspond to at least 10% of the erbiumgain.

An accumulated absorption of about 100 dB at 1530 nm makes it possible,for example, to obtain erbium amplification gain of about 20 dB overband C.

In addition, to obtain 2 dB of Raman gain, for example, with a 1 W pump,it is necessary to have a fiber that is about 1 km long for the leastefficient fiber and about 100 m long for the most efficient. Naturally,a greater length enables greater Raman gain to be obtained withoutrequiring greater pump power.

In a first embodiment of the invention, fiber length is greater than orequal to 100 m, and the absorption maximum is less than or equal to 1dB/m.

The length is adjusted, and more precisely increased, relative to thatof prior art doped-ring amplifying fibers, in order to favor the Ramaneffect. conversely, the absorption maximum is adjusted, and moreprecisely decreased compared with that of prior art doped-ringamplifying fibers.

Nevertheless, Raman amplification of the invention is discrete since itrefers to the Raman effect that occurs over a relatively short length inan amplifying fiber of an amplifier. This amplification differs fromRaman amplification of the kind that is commonly referred to as being“distributed” as is obtained directly in a line fiber and relates to theRaman effect implemented over a longer amplification length.

Preferably, Raman efficiency may be greater than or equal to 3 W⁻¹km⁻¹so as to obtain a fiber that is relatively short.

According to a characteristic, the inner radius of the doped ring may begreater than 1.5 μm so as to adjust the absorption maximum to lie in thedesired range of values.

According to another characteristic, the concentration of active rareearth ions is selected to be less than or equal to 1000 parts permillion (ppm), and when the rare earth ions are erbium ions, less thanor equal to 300 ppm, with concentration in these ranges also making itpossible to adjust the absorption maximum to lie in the desired range ofvalues.

The concentration in active rare earth ions is lower than theconcentration that is usual in prior art doped-ring amplifying fibers.

The selected concentration for active rare earth ions depends on theselected rare earth and on the position of the doped ring. For example,by selecting ytterbium, the maximum acceptable concentration of activerare earth ions is greater than that of erbium. In addition, thisconcentration also depends on the selected length: the greater thelength the smaller the concentration.

Furthermore, the refractive indices selected for the single-mode coreand the multimode core also have some influence. More precisely, for thesingle-mode core having at least a first refractive index and for themultimode core having at least a second refractive index, the differencebetween the first and second refractive indices is preferably greaterthan or equal to 0.01. In addition, the diameter of the single-mode coremay be selected to lie in the range 3 μm to 5 μm.

A refractive index difference that is sufficiently large and a smallcore diameter serve to reinforce the single-mode nature of the centralcore and thus to achieve greater confinement of the fundamental mode ofthe Raman pump wave and of the multiplexed signals injected into thefiber, thereby increasing Raman efficiency.

In a preferred embodiment, the single-mode core is based on silica or onfluoride glass and it is doped by dopants selected from phosphorous,germanium, tellurium, aluminum, and boron. These dopants contribute tohigh Raman efficiency.

In a preferred embodiment, the rare earth doped ring is based on silicaor on fluoride glass and is doped by additional dopants selected fromthe following compounds: Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, BeO, MgO, CaO,SrO, and BaO. These additional dopants increase amplification by rareearth ions.

When the ring is made of fluoride glass, the active rare earth ions arepreferably thulium ions.

Naturally, the present invention also provides an amplifier foramplifying an optical signal, the amplifier comprising:

-   -   a doped ring amplifying optical fiber; and    -   a multimode pump coupled to said fiber perform amplification by        active rare earth ions,

characterized in that the amplifying optical fiber is as defined above,and in that it includes at least one single-mode pump coupled to theamplifying optical fiber to perform Raman amplification in addition tosaid amplification by the active rare earth ions.

In a preferred embodiment, the wavelength of the single-mode pump can beselected to broaden the spectrum of the gain obtained by the active rareearth ions.

In another preferred embodiment, the wavelength of the single-mode pumpis selected to lie in the range of wavelengths in which gain is obtainedby the active rare earth ions. In this manner, the wavelength of thesingle-mode pump can benefit from amplification by the active rare earthions, for example it can be selected to be around 1560 nm for erbium.This amplification of the Raman pump compensates for its loss of energyand enables the Raman gain to be increased.

Depending on the configuration, the Raman gain is involved in a range ofwavelengths having a zone in common with the gain range obtained by therare earth ions, or else situated outside the gain range obtained by therare earth ions. For example, the Raman gain may occur around 1600 nmassociated with amplification by erbium in band C.

Naturally, in this last-mentioned configuration, if optical signalspresenting wavelengths in the amplification range are also injected intothe fiber, then the wavelength of the single-mode pump is selected tolie between two of these channels.

The features and advantages of the invention will appear clearly onreading the following description made by way of non-limitingillustration and with reference to the accompanying figures, in which:

FIG. 1 is a diagram of an amplifier for WDM optical signals in apreferred embodiment of the invention;

FIG. 2 shows the gain profile as a function of wavelength respectivelyfor a prior art EDFA and for the amplifier of FIG. 1;

FIG. 3 a is a fragmentary longitudinal and perspective view of the FIG.1 amplifying fiber; and

FIG. 3 b shows the refractive index profile of the single-mode core andthe multimode core of the fiber as a function of distance x from thecenter of the fiber.

FIG. 1 is a diagram in a preferred embodiment of the invention showingan amplifier 100 for amplifying WDM optical signals s_(u), e.g. in anenlarged band C extending from 1500 nm to 1565 nm.

The amplifier comprises an amplifying optical fiber 1 in accordance withthe invention, having a doped ring with active rare earth ions which arepreferably erbium ions. The amplifying optical fiber 1 of structure thatis described in greater detail below, is in a glass matrix and ispreferably based on silica, possessing a single-mode core and amultimode core. It is long enough to ensure effective discrete Ramanamplification.

The amplifier 100 also comprises:

-   -   a multimode pump 2 for erbium amplification, of power in the        range 1 W to 10 W, and delivering a first pump wave s_(p) of        wavelength equal to 980 nm; and    -   a single-mode pump 3 for Raman amplification, of power        approximately 100 milliwatts (mW) to 5 W, and delivering a        second pump wave s′_(p) of wavelength equal to about 1428 nm,        for example, in order to obtain Raman amplification at the        beginning of band C.

The multimode pump 2 is coupled to a multimode fiber 4 which is itselfcoupled near the inlet 1 a of the fiber 1. In addition, the single-modepump 3 is coupled to a single-mode fiber 4′ which is itself coupled nearthe outlet 1 b of the fiber 1.

In the amplifying fiber 1, the signals s_(u) and the first pump waves_(p) are co-propagating (along the axis in the X direction), whereasthe second pump wave s′_(p) is contra-propagating (along the axis in theopposite direction X′).

In addition, the signals s_(u) and the second pump wave s′_(p) areguided in the single-mode core.

The amplifier 100 is compact since only a single amplifying fiber isused for the two amplification techniques. The amplifier 100 is alsoeconomic in Raman pump power.

In FIG. 2, curves a and b show gain profile as function of wavelengthrespectively for a conventional doped-ring EDFA provided with amultimode pump analogous to the pump 2, and for the amplifier 100.

The gain of the amplifier 100 is clearly greater in the range 1500 nm to1600 nm, and it also enables optical signals to be amplified over abroader range of wavelengths by selecting the single-mode pump to have awavelength of 1428 nm.

FIG. 3 a is a fragmentary longitudinal and perspective view of theamplifying optical fiber 1. FIG. 3 a is diagrammatic and not to scale.

The amplifying optical fiber 1 is of cylindrical geometry, for example,and comprises:

-   -   a single-mode core 10 of small diameter d1, preferably equal to        3 μm, and having a first refractive index n1;    -   a multimode core 20 surrounding the single-mode core 10 and        having a second refractive index n2 that varies, with an outer        diameter d2 equal to about 30 μm, for example; and    -   cladding 30 surrounding the multimode core, with an outer        diameter lying in the range 150 μm to 200 μm, and having a third        refractive index n_(g), where n1>n2>n_(g).

The single-mode core 10 contains germanium dopants 5 increasing theconventional Raman gain obtained by the silica.

Raman efficiency is equal to about 4 W⁻¹km⁻¹.

The multimode core 20 contains a “ring” layer 21 which is doped witherbium ions 6 at a concentration c1. This layer of substantiallycircular circumference presents an inner radius r_(i) and an outerradius r_(e).

The concentration and the position of the ring are adjusted to obtain anabsorption maximum of less than 1 dB/m.

Preferably, the length of the fiber is equal to 500 m, the absorptionmaximum is equal to 0.2 dB/m, and the accumulated absorption is equal to100 dB.

Thus, the concentration c1 is lower than 300 ppm and preferably about100 ppm for Raman amplification. The conversion efficiency for erbiumamplification is high because of the distribution of erbium 6 in thedoped ring.

In addition, the inner radius r_(i) is equal to approximately 4.25 μmwhile the outer radius r_(e) is equal to approximately 6.5 μm.

In FIG. 3 b, curve C shows the profile of the first and secondrefractive indices n1 and n2 as a function of distance x from the centerof the fiber (in microns). More exactly, the ordinate corresponds to thedifference n−n_(g) between the first refractive index n1 and the thirdrefractive index n_(g) and to the difference between the secondrefractive index n2 and the third refractive index n_(g).

The diameters d1 and d2 and the inner and outer radii r_(i) and r_(e) ofthe doped ring are referenced on curve C.

The second refractive index n2 decreases between the inside and theoutside of the multimode core. More precisely, the second refractiveindex n2 remains substantially constant out to about 6 μm, and thendecreases going out to 15 μm. The difference between the first andsecond refractive indices n1 and n2 is always greater than 0.01.

Naturally, the invention is not limited to the embodiment describedabove.

In a variant, the single-mode core 10 contains phosphorous dopantsinstead of (or in addition to) germanium dopants in order to increaseRaman gain, in particular at the bottom margin of band C. In thisvariant, the wavelength of a Raman pump selected accordingly is equal toabout 1305 nm.

In another variant, the wavelength of a Raman pump is not only distinctfrom a wavelength of any optical signal for amplifying, but is alsoselected to lie in the gain range obtained by the active rare earth ionsso as to amplify the Raman single-mode pump wave.

The cross-section of the fiber may present a multimode core of geometrythat is cylindrical, substantially polygonal, or multi-lobed, so as toencourage better absorption of light power by the rare earth doped ring.

The invention is also applicable to band L or any other band.

The fiber may also be made on the basis of fluoride glass.

The erbium-doped ring may contain additional dopants selected from thefollowing: Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, BeO, MgO, CaO, SrO, and BaO.

Finally, any means may be replaced by equivalent means without goingbeyond the ambit of the invention.

1. An amplifier for amplifying an optical signal, the amplifiercomprising: a doped ring amplifying optical fiber; a multimode pumpcoupled to said fiber perform amplification by active rare earth ions,and and at least one single-mode pump coupled to the amplifying opticalfiber to perform Raman amplification in addition to said amplificationby the active rare earth ions; the doped ring amplifying optical fibercomprising: a single-mode core of given diameter; and a multimode coresurrounding the single-mode core and containing a doped layer, having apredetermined concentration of active rare earth ions, the fiber beingconfigured, because of the active rare earth ions, to amplify an opticalsignal for injection into the amplifying fiber; the fiber beingcharacterized in that it is of a length and has Raman efficiency suchthat the product of said length multiplied by said Raman efficiency isgreater than or equal to 0.5 W⁻¹, and less than or equal to 5.0 W⁻¹, andin that, for said fiber presenting absorption for an injected opticalsignal due to the presence of active rare earth ions, said absorptionbeing defined by an absorption coefficient expressed in dB/m andpresenting a maximum value as a function of the wavelength of saidsignal, which value is referred to as the absorption maximum, said fiberpresents accumulated absorption, corresponding to the product of saidlength multiplied by said absorption maximum, of the doped layer of themultimode core which is greater than or equal to 100 dB and less than orequal to 300 dB; wherein a gain is less than or equal to 60 dB.
 2. Anamplifier according to claim 1, characterized in that the wavelength ofthe single-mode pump is selected to enlarge the gain spectrum obtainedby the active rare earth ions.
 3. An amplifier according to claim 1,characterized in that the wavelength of the single-mode pump is selectedto lie in the range of wavelengths in which gain is obtained by theactive rare earth ions.